1. Technical Field
The present invention relates to a communication device and a communication method in a wireless network system such as WLAN (Wireless Local Area Network) of IEEE (Institute of Electrical and Electronics Engineers) 802.11 standard, or WPAN (Wireless Personal Area Network) of EEE 802.15 standard.
2. Background Art
In recent years, attention is being focused on a network shared by wireless terminals with low power consumption, such as WPAN or a sensor network. As a system like system, there is a system called Active RF (Radio Frequency) which sends a radio signal itself.
FIG. 43 is a diagram showing an example of a conventional wireless network configuration.
In FIG. 43, the wireless network includes a control station 1001 which is a wireless control station, and terminal devices 1002, 1003, 1004 which are a plurality of wireless terminal devices. The control station 1001 periodically broadcasts a beacon packet including control information to the terminal devices 1002 to 1004. The terminal devices 1002 to 1004 communicate with the control station 1001 based on the control information.
Various access control methods may be used for the control station 1001 and the terminal devices 1002 to 1004. For example, CSMA (Carrier Sense Multiple Access), TDMA (Time Division Multiple Access) or FDMA (Frequency-Division Multiple Access) may be used.
The wireless devices used in these wireless networks (the control station 1001, and the terminal devices 1002 to 1004) have a feature of low power consumption performance. For example, in order to reduce the power consumption of the wireless devices, a wireless network has a configuration in which an active time interval during which communication is performed in the wireless network, and an inactive time interval during which communication is not performed and the terminal devices may shift to a sleep state are provided. When the inactive time interval is increased, a sleep state can be maintained for a longer time, and thus power consumption may be reduced.
FIG. 44 shows an example of a frame period. Specifically, FIG. 44 shows the relationship between a conventional beacon period, an active time interval, and an inactive time interval.
In FIG. 44, 1 frame period includes an active time interval 1007 and an inactive time interval 1008. The active time interval 1007 is a time interval during which the control station, and the terminal devices 1002 to 1004 communicate with each other.
The inactive time interval 1008 is a time interval during which the control station 1001 and the terminal devices 1002 to 1004 do not communicate with each other, and thus the terminal devices 1002 to 1004 may reduce the power consumption by shifting to a sleep state. Even in the active time interval 1007, a terminal device which does not perform communication may reduce the power consumption by shifting to a sleep state.
The control station 1001 and the terminal devices 1002 to 1004 share and use the active time interval. The control station 1001 uses an initial portion of the active time interval to broadcast a beacon frame 1009. That is to say, the beacon period 1006 is the sum of the active time interval 1007 and the inactive time interval 1008.
The active time interval except the time interval during which a as beacon frame is broadcasted is used for communication between the control station 1001 and the terminal devices 1002 to 1004, and for example, CSMA may be used. The active time interval may be divided into a plurality of time slots, and slots may be shared and used by Slot CSMA or TDMA. For example, in the IEEE 802.15.4 standard, the first half time slot of the active time interval is used for competition access by CSMA, and the second half time slot of the active time interval is used for communication where each time slot is assigned to a wireless device which may be used during the time slot.
The beacon frame contains control information related to a frame, such as the number of these time slots, their assignment, the length of the active time interval, the length of the inactive time interval, and the time until the subsequent beacon frame is transmitted.
FIG. 45 is a flowchart showing an example of a sequence of data communication from the conventional control station 1001 to the terminal device 1002.
As shown in FIG. 45, when data to be transmitted to the terminal device 1002 is generated (S1010), the control station 1001 buffers the data. The control station 1001 adds information as buffered data to a beacon frame, the buffered data to be transmitted to the terminal device 1002. For example, in the IEEE 802.15.4 standard, the control station 1001 adds Data Pending Address List to the beacon frame, and broadcasts the beacon frame at the initial time of the active time interval (S1011). However, in IEEE 802.11 standard, the control station 1001 adds TIM (Traffic Indication Message) to the beacon frame instead of Data Pending Address List, and broadcasts the beacon frame at the initial time of the active time interval.
The terminal device 1002 shifts from a sleep state to an active state at the timing when receiving a beacon frame, and then receives the beacon frame (S1011). The beacon frame contains information such as the length of the active time interval, the length of the inactive time interval, and the time until the subsequent beacon frame is transmitted, Data Pending information. The terminal device 1002 shifts to an active state at the timing when receiving a beacon frame without fail, and then receives the beacon frame.
The terminal device 1002 analyzes the Data Pending information of the beacon frame (S1012). The terminal device 1002, when recognizing that there is data addressed to the self device, transmits a data request to the control station 1001 (S1013). The control station 1001, when receiving a data request (S1013), transmits the buffered data to the terminal device 1002 (S1014). The terminal device 1002 returns ACK which is an arrival acknowledgement signal to the control station 1001 (S1015).
As described above, the terminal device 1002 shifts from a sleep state to an active state at the timing when receiving a periodically transmitted beacon frame, and checks whether or not there is data addressed to the self device based on the beacon frame. When there is data addressed to the self device, the terminal device 1002 inquires about the data to the control station and receives the data. The terminal device 1002 shifts to a sleep state and keeps the state until subsequent timing of receiving a beacon frame. On the other hand, when there is no data addressed to the self device, the terminal device 1002 immediately shifts to a sleep state and keeps the state until subsequent timing of receiving a beacon frame. By these operations, low power consumption of the terminal device 1002 is achieved.
That is to say, in order to achieve significantly low power consumption of the terminal device 1002, a method may be considered in which the time during which the terminal device 1002 is in a sleep state is increased, and the period of the beacon frame is increased. However, while the low power consumption of the terminal device 1002 may be achieved, the buffering time in the control station 1001 increases, and thus the delay time until the data is delivered to the terminal device 1002 increases. This presents a problem for application for which real time performance and immediacy are demanded.
For the above-mentioned problem, a method disclosed in Patent Literature 1 collects statistics on the past record of the time information of the data frame which is received by the terminal device, and then determines whether the data traffic is periodical (stream) or continuous (burst). When the data received by the terminal device is periodical (stream) data, the transmission period of the beacon frame from the control station is changed in accordance with the reception interval of the data. On the other hand, when the data received by the terminal device is continuous (burst) data, a time interval during which data is continuous, and a time interval during which no data transfer is found for the time being are distinguished, and it is determined whether an active state is continued or a sleep state is activated according to the distinguished time interval. Accordingly, in Patent Literature 1, delay of data reception is reduced, real time performance is maintained, and improvement of power saving effect is achieved.